Automotive engines have been developed to obtain higher horsepower in a more compact package with reduced weight. The increased horsepower and weight reduction have been achieved simultaneously with cleaner, more efficient burn and less particulate and gaseous emissions. Lower maintenance is required with modern day automotive engines. Oil changes and spark plug changes now occur at larger mileage intervals as compared to twenty years ago. Fuel efficiency has increased to provide greater mileage per gallon.
The increased horsepower in smaller packages has made it possible to reduce engine compartment sizes in small automobiles so that comfortable and relatively spacious passenger compartments can be provided. The trend has been to make even smaller engines to complement the smaller automobiles allowing even more space for passenger compartments.
While smaller engines have been developed with sufficient horsepower for highway speeds and adequate acceleration, they are more sensitive to the power drain from accessories such as cooling fans, air conditioning, automatic transmission, oil pumps, and four wheel drive. Often the power drain is sufficiently great that the acceleration during the power drain becomes unacceptable. As a result, clutches for accessories have been developed so that the accessories are not continuously and needlessly operated. Often, the clutches are designed to disconnect accessory units during acceleration or other high load conditions to lower the maximum load or torque of the engine.
Smaller batteries have also been developed to accompany the smaller engine compartments to provide starting cranking power. Often these smaller batteries have less ampere-hours and less power than larger heavier batteries and may be more easily drained if there is a problem starting the automotive engine.
Oil pressure is provided by an oil pump to circulate the oil through the engine block to lubricate the various moving parts such as the camshaft, crankshaft, pistons, and connecting rods and other various bearings. The lubrication is necessary to prevent metal to metal contact. The oil is circulated to replace the oil that is warmed and expelled from the bearings due to the heat emanating from the bearings and lubricated surfaces. If the oil is overheated, the oil viscosity becomes very low and the oil breaks down thereby allowing metal-to-metal contact between the bearing surfaces. A gear or chain driven oil pump has been traditionally used by the engine to provide flow of the oil at all times.
When the engine is fully warmed up and is at operating temperatures, the oil is hot. Consequently, the viscosity is lower, and oil flows more freely through the various bearings. Since the necessary flow of oil depends on pressure, viscosity and clearance (i.e. passage restriction), and the clearance is virtually a constant, the flow increases as the viscosity decreases and the pressure drops as the oil is warmed. Under such conditions, in order to maintain the necessary flow of oil for keeping the bearing cool enough to establish the required hydrodynamic lubrication film thickness, the pressure must be increased. That means that the size and capacity of the oil pump must be sufficient or in excess of the maximum requirement after the engine is used and partially worn, to assure the necessary flow to keep the bearings from oil starvation and mechanical failure under the worst conditions of temperature (hot) and mechanical wear (increased clearances). The oil pressure by being dependent on the speed of the oil pump, is dependent on the engine speed up to a point where a pressure regulator valve open at or near 60 psi on many engines.
When the engine is cold such as during a cold start, the oil is cold and the viscosity is high, the oil pressure provided by the oil pump is highest. However, this high oil pressure is not desired nor is it beneficial. The high viscosity and the temperatures of the cold oil makes it unnecessary for the oil pump to circulate much oil. Secondly, the oil pressure exerted by the oil pump provides resistance to the cranking of the engine during start up which provides an unnecessary power drain on the battery, lower cranking speeds and increased fuel consumption during cold operation.
All engines use a pressure-relief valve to limit the maximum line oil pressure, for the purposes of providing excess capacity to compensate for hot-oil operating conditions, and for oil which is too thin or diluted by fuel contamination and excessive clearances in the bearings. This pressure-relief valve in many cases is not sufficiently large to handle the large pressure increases at the pump head with cold oil, i.e., when the natural flow of oil past the bearings is limited by the high viscosity of the cold oil; the efficiency of the pump itself is also increased due to the reduced internal leakage. The pressure increase is further compounded by the engine being run at a high speed to promote quick engine warm-up. The applicant is aware of cases where the pump's driving torque under such conditions has been as high as eight times the normal running torque, and where very expensive failures have occurred due to oil pumps drive failure. In such cases, it has been demonstrated that dumping as much as 75% of the pump's capacity, and dumping it at the pump head, solves the problem without other detrimental effects.
Systems have been developed to vary the oil pressure dependent on the operating temperature of the engine. Some prior art systems use two pressure-operated relief valves, one at the pump discharge. Other which use multiple oil pumps provide an electrically operated on-off solenoid valve to dump the output of one of the pumps. For the larger type of automotive engines and large truck diesels operating in very cold regions, very expensive and complicated engine cranking systems are used to generate enough speed and rapidity of compression to initiate combustion. Since the oil pump in such cases requires so much torque, and since the bearings constantly being fed cold oil also increases their own oil-shear friction, dumping the line pressure during cranking in such conditions allow more cranking torque to be used to turn the engine over (crank it) faster, greatly increasing the engine's starting ability. What is needed is a economical system that reduces the pressure and flow of oil during cold engine conditions and controls the oil pressure as the engine temperatures increase.